A major area of application of tube bundles is in shell-and-tube heat exchangers. A shell-and-tube heat exchanger comprises a cylindrical vessel in which a bundle of parallel tubes are arranged extending in longitudinal direction of the vessel.
As is well known, the shell-and-tube heat exchanger is an indirect heat exchanger in which heat is transferred between a fluid passing through the tubes of the tube bundle (the tube side) and a fluid passing through the space outside the tubes (the shell side). Details of the shell-and-tube heat exchangers can for example be found in Perry's Chemical Engineers' Handbook, 6th edition, 1984, McGraw-Hill Inc., page 11-3 to 11-21. The ends of the tubes of the tube bundle are secured to a tube sheet. The heat exchanger can include two tube sheets, one at each end of the cylindrical vessel, or a single tube sheet at one end of the cylindrical vessel in the event the heat exchanger is a U-tube exchanger.
The tubes need to be supported. This can be done by axially spaced apart transverse supports.
It will be understood that the intermediate portions of the tubes have to be supported as well, for example to prevent damage to the tubes due to vibrations caused by the fluid flow. To support the intermediate portions of the tubes, a support can be used including transverse support plates that are spaced apart along the length of the tubes.
A conventional support comprises segmental baffles, and there are several kinds discussed in Perry's. Baffles do not only support the tubes, but also influence the fluid flow through the shell side. Therefore the design of a baffle is as well determined by heat-transfer considerations.
Another type of tube support is formed of rods or bars arranged in lanes between the rows of tubes. A transverse support consists of a support ring that has an outer diameter that is somewhat smaller than the inner diameter of the cylindrical vessel and parallel rods or bars that are secured at their ends to the support ring.
Tubes of a tube bundle are most commonly arranged on an equilateral triangular pitch (wherein the tubes are so arranged that their centres are on the corner points of equilateral triangles). A square pitch (wherein the tubes are so arranged that their centres are on the corner points of squares) is also used, often for mechanical cleaning purposes in removable-bundle exchangers.
U.S. Pat. No. 4,143,709 discloses a support for a bundle of parallel tubes, which support comprises a plurality of transverse support plates spaced apart along the length direction of the tubes to be supported. Each plate is integrally formed from a single sheet wherein a plurality of holes is cut on a regular grid, each of which holes is large enough to accept a plurality of tubes. The holes can be generally rectangular-shaped, generally square-shaped, generally triangular-shaped or generally rhombical (diamond-shaped).
At least one of the support plates of the known support is disposed out of phase or staggered. Tubes extending through the same hole in one support plate extend through different holes in another support plate, so that the cooperating adjacent plates support the tubes from opposite sides against lateral movement.
The generally triangular-shaped or generally diamond-shaped embodiments are adapted to support tubes on a triangular pitch. However, in these embodiments the width of the cross-laths of the supporting grid must be made significantly less than the shortest distance between adjacent tubes, which is not optimal for reasons of mechanical strength.
Applicant has found that tube supports can very advantageously be manufactured using expanded metal, and this is the subject of International Patent Application No. PCT/EP03/01074 in the name of Applicant, which was unpublished at the first filing (priority) date of the present application.
A sheet of expanded metal is made from sheet metal that is slit according to an arrangement of staggered parallel dashed lines, and stretched (expanded) perpendicular to the dashed lines into a structure of cross laths with interstices. Its manufacture is simple and cheap. Moreover the sheets can easily be cut to a desired shape for a particular application, e.g. circular shape, for example by laser cutting.
The openings of expanded metal have substantially rhombical or frustro-rhombical shape. Substantially rhombical shape (the shape of an equilateral parallelogram) is obtained when the slits along a virtual dashed line are much longer than the non-slit parts (so-called bonds) between them. After stretching of the slit sheet metal, the bonds form corner points each connecting four so-called strands of equal length. If the non-slit parts between the slits are relatively long, than a substantially frustro-rhombical shape is obtained. By frustro-rhombical it is meant that two opposite corners of a rhombical shape are cut off along a pair of parallel lines. Known expanded metal is stretched to a maximum of 90 degrees, resulting in approximately square openings, as can for example be obtained from Sorst Streckmetall GmbH, Hannover, Germany.
The strands after stretching of expanded metal are not perfectly straight but often slightly S-shaped; i.e. curved at the connections with the bonds, with a substantially linear central part in between. It will be understood that the V-shape of pairs of adjacent (neighboured) strands in the direction of stretching is somewhat distorted. The stretching angle is suitably defined by the central parts of the strands forming the (distorted) V-shape.
Also, when expanded metal is stretched out, the strands are twisted out of the plane of the sheet metal, unless the sheet is subsequently flattened again. The slight S-shape of the strands and their attachment to the bonds has the consequence that the openings have no more than two mirror symmetry axes, which can be along and perpendicular to the direction of the stretching. This will be discussed in more detail with reference to the drawings.
To account for deviations of the openings in expanded metal from a perfect rhombical or frustro-rhombical shape are, the term ‘substantially rhombical or frustro-rhombical shape’ is used in the description and in the claims. The term therefore encompasses perfect and imperfect or distorted such shapes.
Applicant has found that the deviations from perfect rhombical or frustro-rhombical shape make it difficult to use expanded metal in a staggered arrangement like in U.S. Pat. No. 4,143,709, since at least two different types of passageways are formed with slightly different cross-section and shape, which is undesirable for optimum tube support.
It is an object of the present invention to provide an improved support for a tube bundle, comprising at least two transverse support plates provided with a plurality of openings, wherein openings in different support plates are arranged to form support passages for parallel tubes so that support passages extending through the same opening in one support plate extend through different openings in another support plate. It is a particular object to provide such a support that provides optimised mechanical stability. It is another particular object of the invention to provide such an improved support that can be made of expanded metal.